Abstract: A photometer including an elongated sample cell having a first end, a second end, and a passageway extending between the first end and the second end. Preferably, a ratio of a length of the sample cell to a lateral dimension of the passageway is at least 100 to 1. A first quartz window assembly is located at the first end of the sample cell and has a first port communicating with the passageway proximate to the first end, and a second quartz window assembly is located at the second end of the sample cell and has a second port communicating with the passageway proximate to the second end. An ultraviolet lamp is positioned to emit ultraviolet light through the first quartz window, the passageway, and the second quartz window, and an ultraviolet detector is positioned to receive the ultraviolet light emanating from the second quartz window. Preferably, the sample cell is operated at about ambient temperature, and the volume of the sample cell is no greater than about 0.